US prosecutors late on Wednesday filed a lawsuit to seize Iranian fuel from four tankers carrying it to Venezuela.
It is the latest attempt by the US administration of President Donald Trump to increase economic pressure on the two foes.
The government of Venezuela's socialist President Nicolas Maduro has made no secret of the tankers, which departed last month, to show it remains unbowed by US pressure.
Washington has been pressing for the removal of Mr Maduro with measures including sanctions on state oil company PDVSA.
Petrol shortages in Venezuela, which like Iran is a member of Opec, have grown acute due to US sanctions, and the country has undergone an economic collapse.
Its refineries are largely inactive through neglect and lack of investment.
But Mr Maduro has held on and the failure to unseat him has frustrated the US president.
In the civil-forfeiture complaint, US federal prosecutors aim to stop the delivery of Iranian fuel aboard the Liberia-flagged Bella and Bering, and the Pandi and Luna, according to the lawsuit, first reported in The Wall Street Journal.
The suit, filed in the US District Court for the District of Columbia, seeks to deter future deliveries.
US District Judge James Boasberg issued a warrant for the seizure of the more than 1.1 million barrels of petrol in the four tankers, the Justice Department said on Thursday.
The warrant allows American authorities, probably the US Coast Guard, to seize the fuel.
The lawsuit also aims to stop the flow of revenue from petroleum sales to Iran, which Washington has sanctioned over its nuclear programme, ballistic missiles, and influence across the Middle East.
Tehran says its nuclear programme is for peaceful purposes.
Zia Faruqui and two other assistant US attorneys allege in the lawsuit that Iranian businessman Mahmoud Madanipour, who is affiliated with Iran's Islamic Revolutionary Guard Corps, helped to arrange the shipments by changing documents about the tankers to evade US sanctions.
The lawsuit says that since September 2018, the Guards' elite Quds Force has moved oil through a sanctioned shipping network involving dozens of ship managers, vessels and facilitators.
Profits from the shipments support the "full range of nefarious activities, including proliferation of weapons of mass destruction and their means of delivery, support for terrorism, and a variety of human rights abuses, at home and abroad", the lawsuit said.
The ships carrying Iranian petrol engaged in ship-to-ship transfers to evade sanctions, the lawsuit said.
It was not immediately clear whether or how the US would move to seize the petrol.
The US government must prove the fuel is forfeitable under law in a civil proceeding before seizing it permanently.
Last year, the US failed to stop the Adrian Darya, formerly known as Grace 1, a tanker in the Mediterranean carrying Iranian oil, through blacklisting it and other measures.
The Adrian Darya was originally seized by British Royal Marine commandos on suspicion of being on its way to deliver to Syria against UN sanctions.
But it was released by Gibraltar after receiving written assurance from Tehran the ship would not deliver its cargo of 2.1 million barrels of oil to the regime of Syrian President Bashar Al Assad.
After a warrant was issued for the Adrian Darya last year, Brian Hook, the State Department's top Iran official, sent emails to its captain.
They said the Trump administration was offering him several million dollars to steer the tanker to a country that would impound it on behalf of Washington.
The oil was eventually sold to Syria.